Filter cartridges have been used more and more frequently in industrial dedusting processes during recent years. The filter cartridges generally consist of: a supporting grid; a filter medium in the form of a pleated bellows, which is arranged around the supporting grid in the form of a cylinder; a lower closed end piece; and an upper end piece with a central outlet. The bellows is attached to the two end pieces so as to avoid dust leakage.
It is state of the art to use mainly filter paper or thermally bonded non-wovens as filter media for such filter cartridges. It is most important that the filter material, pleat depth, and the number of pleats interact to accommodate a sufficiently large filter surface in a filter cartridge. On the one hand, the filter material must be rigid enough to prevent the pleats from clinging together or lying directly on each other during the cleaning process, which might considerably reduce the effective filter area. On the other hand, the material must be flexible enough so that cleaning is supported by the relative movements between the surface of the filter medium and the dust layer and that the forces impacting the filter cartridge be absorbed without damage to the filter medium.
For this purpose it is state of the art to hold the folded filter material stationary by various spacer constructions. It is essential to ensure the correct distances between component parts because any clinging together of the pleats of the pleated bellows would reduce the filter surface, worsen separation efficiency, and lead to increased pressure losses due to insufficient cleaning of the remaining filter area.
Furthermore the state of the art includes a method wherein filter paper (which is frequently used because it is relatively stiff) is provided with a spacer function by embossing the filter paper (e.g., during the pleating process). After this process, the filter paper is hardened, e.g., in an oven, which leads to a permanent embossing effect in the material. Then the filter paper is built into the cartridge such that the raised embossed parts of every two pleats of the pleated bellows are in contact with each other, which results in a spacer effect.
However, certain shortcomings have been experienced with the embossing of filter paper. Above all, the filter paper is heavily stressed at the embossed spots so that minor material variations entail a danger of faulty "buds" that can fatally damage or severely shortening the service life of a new filter cartridge.
Other filter materials may be used for the above mentioned filter cartridges, e.g., thermoplastic materials or non-wovens made from synthetic fibers. Here, too, measures must be taken to ensure the required spacing effect.
As has been determined, the embossing described above for paper filters cannot be performed on filter media consisting of mechanically sensitive materials, such as expanded PTFE, because wear and tear occurs due to friction between the individual pleats of the pleated bellows at the points where such raised parts or buds touch each other. This condition negatively affects the retention capacity of the filter medium in such areas.